1. Field of the Invention
The present invention relates generally to the management of radio frequencies in a wireless mesh network (WMN), that may however have one or more wired links thus forming a hybrid network.
For the sake of brevity, WMN will be mostly referred to hereinbelow.
More in particular, the present invention relates to communication nodes, a communication system, and a method of managing radio frequencies within a wireless or hybrid mesh communication system.
In the present description and in the attached claims, “communication node” is used to indicate a single entity from a logical point of view, but may refer to a single device, a group of devices, or a portion of a device from a physical point of view.
2. Description of Related Art
It is increasingly common to interconnect several fixed and mobile electronic appliances, such as computers and peripherals therefor, handheld digital devices like personal digital assistants and cell phones, data collection devices and sensors for example for area video surveillance, and so on. Said devices, as well as dedicated network apparatus or access points coupled to the devices, comprise the nodes or stations or units or terminals of the communication system. Term network will be used herein as a synonymous of term communication system.
The connection or links among the nodes may exploit several physical media, such as electric wires, buses and optical fiber, as well as wireless media such as radio frequency and infrared electromagnetic radiation.
While wired communication systems are still broadly used, wireless and hybrid type communication systems are increasingly common due to their flexibility of installation, allowing for mobile devices to interconnect with each other and/or with an infrastructure.
WMNs, when operating in the 5 GHz band, should comply with Dynamic Frequency Selection (DFS) requirements in order to give priority to radar transmission. Such requirements are presently dictated, in Europe, by ETSI standard EN 301 893, and in the U.S. by FCC rules, Title 47, Part 15.
The basic DFS requirements are that, in case a radar signal is detected on a given channel or frequency—referred to as a DFS event —, the node(s) operating on said channel must promptly clear the channel and refrain from using it for a specified time after the DFS event; and that before (re)using a channel, the node(s) must monitor it for ensuring absence of radar signals.
More in particular, the standards presently define the following quantities and figures, which are provided hereinbelow just as a non limiting example:                Channel Availability Check Time TCAC: the time a system shall monitor a channel for presence of radar prior to initiating a communications link on that channel. Amounts to 60 seconds outside 5600-5650 MHz or 10 minutes for 5600-5650 MHz sub-band.        Channel Move Time TCM: the time for the system to clear the channel and measured from the end of the radar burst to the end of the final transmission on the channel. Amounts to 10 seconds.        Channel Closing Transmission Time TCCT: the total, or aggregate, transmission time from the system during the channel move time. Amounts to 1 second in ETSI regulation.        Non-Occupancy Time TNO: a period of time after radar is detected on a channel that the channel may not be used. Amounts to 30 minutes.        
A practical implementation of a wireless or hybrid mesh network compliant with such requirements cannot refrain to communicate for the entire TNO, and must therefore provide for switching to another channel upon detection of a DFS event. The standards themselves do not provide for any indication of how to perform channel changes.
WO 2009/145930 A2 and US 2009/0201851 A1, assigned to Cisco Technology, Inc., and entitled “Coordinated Channel Change in Mesh Networks”, disclose two methods to reconfigure channels frequencies in the WMN in a distributed and coordinated manner and it explicitly considers the radar interference scenario. The considered WMN topology is a tree where the root mesh access point serves as a central point which coordinates the channel reassignment operations. Radar events can be detected at any node in the network and are notified to the root. One of the change methods defines a “bottom-up” procedure where the root node sends an acknowledged prepare-to-change message to child nodes which in turn forward it until the leaves are reached. Then, a ready-to-change message is sent from the leaves to the parent node, and an acknowledged change-to-channel is sent from the parent node to the leaves. A parent node performs the change only when all of its children have completed the reconfiguration, and so on until the root node is reached. In the other method, the mesh nodes reconfigure their radios in a “top-down” fashion, where parent nodes forward a channel change message (originated from the root) to their children and after that perform the channel change; each message is sent multiple times to each child to increase robustness.
WO 2006/110404 A2, assigned to Interdigital Technology Corporation, and entitled “Method and apparatus for coordinating seamless channel switching in a mesh network” also covers the radar interference scenario. It defines a set of procedures to a) elect a Channel Master node at the beginning of network operation which is responsible for coordinating a set of Mesh Points (MP) operating on the same channel; b) collect radio measurements, including radar pulse events, from the different nodes in the network using layer-2 messages, which are stored in a dedicated information base which also includes channel assignment policies, allowed spectrum bands, timing settings, etc; c) broadcast channel-change messages to set of managed nodes, and receive the respective responses. More specifically, when detecting a radar, the Mesh Points report the trigger condition to the Channel Master and/or to neighbor MPs and wait for a predetermined amount of time for a reply initiating a frequency change from the CM. The CM sends a mesh channel switch announcement (MCSA) either to all or a subset of MPs under its responsibility. The MCSA signaling can affect only one particular mesh-link, a group of mesh-links, or change the settings of all MPs. The MPs which have received the MCSA will change their frequency settings according to the information received by a channel change confirmation message. They may or may not acknowledge successful reception or execution of the changes in the channel change confirmation message to the CM.
The above document also considers a fully distributed implementation where each MP in the network operates independently without the need of CM nodes. In this case, all nodes exchange information about their frequency assignment capabilities and independently take actions to switch channels basing on the data received from the neighbor nodes.
US 2005/0192016 A1, assigned to Telefonaktiegolaget LM Ericsson (publ), and entitled “Method and device for controlling frequency selection within a wireless communication system” discloses continuously or quasi-continuously monitoring and assessing a plurality of frequencies with respect to a radar-like interference signal and allocating a quality parameter to each assessed frequency, the quality parameter indicating the probability that the frequency is occupied, and selecting one or more frequencies in dependence on the allocated quality parameters and subsequent frequency monitoring. The monitoring device may be part of an Access Point or a Central Control or be located remote from but in communication with the AP or CC. The quality parameters can be communicated to further APs or CCs.
US 2004/0156336 A1, assigned to Atheros Communications, Inc., and entitled “Methods for implementing a Dynamic Frequency Selection (DFS) and a temporary channel selection feature for WLAN devices” discloses WLAN devices and techniques in WLAN devices for performing startup scans for radar, identifying backup channels for a channel switch, and efficiently changing channels in the event of radar detection in the operating channel. During startup, the access point can quickly scan multiple channels for radar. In one embodiment, normal operation can begin on the first channel that is found without radar. During normal operation, the access point or one or more stations at the request of the access point can perform short background scans to identify additional radar-free channels. The scanning process can continue until one or two backup radar-free channels are found. At this point, the radar-free channel(s) can be stored in the access point for use during a future channel switch event. The document provides i.e. for setting, based on radar detection, a new channel selected from a previously-scanned channel list; selecting a temporary channel exempt from radar scan and recommencing normal operation using the temporary channel; performing an aggregate background scan on the new channel; and if the new channel passes the aggregate background scan, then recommencing normal operation using the new channel.
US 2009/0116400 A1 to Sekiya, and entitled “Wireless communication apparatus and wireless communication method enabling switching of communication channel” discloses that an access point transmits an instruction frame to a wireless communication terminal when a monitoring unit detects a radar or interference signal in a first communication channel. The instruction frame includes information on a second and a third communication channel, and instructs the wireless communication terminal to switch the communication channel from the first communication channel to at least one of the second and third communication channels.
The Applicant faced the problem of providing a management of radio frequencies that in particular complies with DFS requirements, and provides high performance in the reconfiguration of the network after detection of a DFS event.
The invention is however applicable more in general, whenever there is a need or a desire to move from one frequency channel to another one, for example because of temporary unavailability of the channel, low performance of said channel, high interference, etcetera.
Secondary objects of the present invention are possibly to provide such a frequency management while maximizing the preservation of the existing network topology, avoiding changes in the routing status, and minimizing overall network interference.